The present invention generally relates to wireless communications networks, and more particularly relates to techniques for reducing interference in heterogeneous cell deployments where transmission nodes have different transmission powers and overlapping coverage areas.
An interesting deployment strategy for cellular networks is the use of a so-called heterogeneous deployment or heterogeneous network. A heterogeneous network consists of several network transmission nodes having different transmit powers and operating with overlapping coverage areas. FIG. 1 illustrates an example of one such deployment, where macro nodes 110 each provide traditional cellular coverage over a wide area while pico nodes 120 provide “spot” coverage at various places within the macro coverage area.
In such a deployment, the low-power nodes (“pico nodes”) are often deployed to offer high data rates (e.g., Megabits/second), as well as high capacity (e.g., in terms of users/meter2 or Megabits/second/meter2), in the local areas where high capacity and/or high-data rates are needed or desired. The high-power nodes (“macro nodes”) provide full-area coverage. In practice, the macro nodes may often correspond to currently deployed macro cells while the pico nodes are later deployed nodes, extending the capacity and/or achievable data rates within the macro-cell coverage area where needed.
Transmissions to a mobile terminal (a user equipment, or “UE,” in 3GPP terminology) are often divided into control plane and user plane transmissions. Examples of control plane signaling include radio-resource control messages used for mobility and system information necessary for the terminal to access the network. The user plane, on the other hand, refers to transmissions of the actual data for the end user or end application.
A pico node in some heterogeneous deployments may correspond to a cell of its own (a “pico cell”). This means that the pico node has its own identity, and a corresponding cell identifier, and transmits the full set of common signals/channels associated with a cell, in addition to performing downlink and uplink data transmission/reception. In an LTE context, these associated signals and channels include the primary and secondary synchronization signals, cell-specific reference signals, and system information related to the cell. Thus, both the user plane and control plane information are handled by a single pico node.
In a different approach, a pico node does not correspond to a cell of its own, but is instead used primarily for user-plane transmission. In this type of deployment, a mobile terminal served by the pico node relies on the macro layer for at least parts of the control plane signaling. This latter approach has several benefits, such as improved mobility robustness and improved energy efficiency. Since the macro layer is responsible for providing parts of the control-plane information, e.g., by broadcasting system information, the pico node only needs to be active when actually transmitting user data to the terminal. This can lead to significant gains in energy efficiency and an overall reduction in interference, since the pico nodes can be silent in periods of no data transmission activity.
The two general approaches described above are illustrated in FIGS. 2A and 2B. FIG. 2A illustrates a scenario in which the user plane and control plane for a given mobile terminal 230 are both terminated at a single node, in this case at the pico node 120. In FIG. 2B, mobile terminal 230 instead exchanges control plane information with macro node 110, while exchanging user plane data with pico node 120.
Of course, the control and user planes could easily be further subdivided between the macro and pico nodes in different ways. For example, transmission of control and user data could be split according to different sets/types of information, such as by transmitting critical control plane signaling and high-quality user plane transmissions (voice call) from the macro node on an “anchor carrier” and less critical control signaling and/or user data from the pico node on a “booster carrier”. In this scenario, the anchor carrier would provide basic connectivity and handle mobility while the booster carrier, when present, would boost the data rates for the terminal. This approach may be generalized to more than two sets of information and more than two sets of nodes.